Conventionally, in an engine for an automobile or a like vehicle, for instance, there is employed a technique for supplying engine oil from an oil pump to each part of the engine for lubricating bearing portions and sliding portions, for cooling pistons, or for supplying operating hydraulic pressures to various devices.
Generally, a requested hydraulic pressure of engine oil differs depending on operating conditions of an engine (such as a rotation speed, a load, and an oil temperature). For instance, when the oil temperature is high, the amount of oil leaking from a bearing portion may increase, which may make it difficult to raise the hydraulic pressure. In view of the above, it is necessary to keep the hydraulic pressure relatively high, as the oil temperature increases. Further, as the rotation number of an engine increases, the amount of engine oil required for cooling pistons increases. In view of the above, it is necessary to increase the hydraulic pressure, as the rotation number of an engine increases. Furthermore, a variable valve timing mechanism (hereinafter, abbreviated as VVT) and a valve stop mechanism for a reduced cylinder operation are switched between an operative state and an inoperative state depending on an operating condition of an engine. In view of the above, it is necessary to change the hydraulic pressure, each time a switching operation is performed.
Supply of engine oil in excess of a required amount and pressure, however, may increase driving loss of the oil pump, and deteriorate the fuel economy of the engine. Therefore, in order to increase the fuel economy, there is a need for a technique for appropriately controlling the amount and pressure of oil to be supplied depending on an operating condition of an engine.
For instance, Patent Literature 1 discloses a technique, in which a hydraulic control valve (a duty linear solenoid valve) is provided in a discharge passage of an oil pump to control the hydraulic pressure of engine oil to be supplied to each part of an engine depending on an operating condition of the engine.
In the aforementioned technique described in Patent Literature 1, however, the oil pump is of a fixed capacity type. When the requested hydraulic pressure (oil amount) is small, engine oil that is discharged from the oil pump is fed back to an oil tank by the hydraulic control valve. Consequently, work of the oil pump when the engine oil, which is resultantly fed back, is discharged from the oil pump is useless, and the fuel economy effect is low.
Further, for instance, Patent Literature 2 discloses a technique, in which an oil pump of a variable capacity type is used as an oil pump for supplying an operating hydraulic pressure at which a variable lift mechanism of intake and exhaust valves is operated, and a requested discharge amount for obtaining requested lift characteristics of the valves is determined from an engine rotation speed, an engine load, and an oil temperature for controlling the discharge amount of the oil pump based on the total requested discharge amount.
The aforementioned technique described in Patent Literature 2, however, does not satisfy requested hydraulic pressures of the hydraulically operated devices at the same time. Further, the aforementioned technique is not directed to feedback controlling a hydraulic pressure based on a detection value. Therefore, precision of capacity control of the oil pump is low. Consequently, the fuel economy effect is insufficient.